Its a Hamamatsu P1501 Dip 5 opto-coupler device. Try here http://www.chinadatasheet.com/download.asp?id=1376323_852681. This device is older than the hills and probably obsolete. The resistance across pins 4 and 6 should drop as the voltage across pins 1 and 2 increases (I think!!!).
I was wondering if anyone has a datasheet or pinout for the PC-1 optocoupler, i'm thinking mine is broken but i can't verify it as i can't really discern the pinout from the schematics.